1. Field of the Invention
The present invention relates to exposure apparatus and method used in a lithography process for manufacturing, for example, semiconductor devices, liquid crystal displays, plasma displays, thin film magnetic heads, image pickup devices (such as CCD), and photomasks (reticles). In particular, the present invention is concerned with scanning exposure apparatus and method wherein a photomask and substrate are moved relative to each other with respect to an exposure beam.
2. Related Background Art
In a lithography process for manufacturing, for example, semiconductor devices (such as LSI), step-and-repeat type reduction projection exposure apparatuses, or so-called steppers, and step-and-scan type reduction projection exposure apparatuses, or so-called scanning steppers, have been mainly used. In recent years, scanning steppers, which can more easily achieve high-accuracy large-area exposure than steppers, have been dominantly used in order to meet increasing fineness or smaller feature size, and increasing size of semiconductor devices.
In the exposure apparatus as described above, a reduced image of a device pattern is projected onto a wafer through a projection optical system, while alignment marks are detected by an off-axis alignment system located on one side of the projection optical system. Reasons why this alignment system is used will now be explained.
The projection optical system has image-forming characteristics (for example, aberration) that are suitably adjusted according to the wavelength of illumination light for exposure. Where an on-axis type alignment system is employed which detects alignment marks on a wafer via the projection optical system, therefore, the system must use alignment light whose wavelength is close to the exposure wavelength. As the wavelength of the illumination light for exposure gets shorter and shorter, it becomes considerably difficult to select light to which a resist on the wafer is not sensitive, and which causes only small chromatic aberration in the projection optical system. For this reason, the off-axis type alignment system has been increasingly employed which is able to determine the wavelength or width of the alignment light irrespective of the exposure wavelength. In the meantime, an automatic focusing sensor is also provided on one side of the projection optical system.
In a known exposure apparatus, an L-shaped alignment optical system unit with a bent optical axis is disposed at one side of an optical element that is located closest to the wafer in the projection optical system. Thus, the distance between the center of detection of the alignment optical system unit and the position at which a reticle pattern is projected (and which is typically represented by the center of the optical axis of the projection optical system), i.e., the baseline distance, can be reduced as compared with the case where a cylindrical alignment optical system unit with a straight optical axis is provided. The above L-shaped arrangement that can reduce the baseline distance has been generally used for improved stability of the mark detecting accuracy (alignment accuracy) that depends upon the baseline distance.
With the L-shaped arrangement as described above, however, the alignment optical system cannot provide a large numerical aperture (N.A.), thus making it difficult to improve the mark detection accuracy. Namely, in order to improve the resolving power of the exposure apparatus, the N.A. of the projection optical system needs to be increased to, for example, 0.6 or larger, resulting in an increased aperture of the projection optical system. Even if the L-shaped arrangement as described above is employed, it is extremely difficult to locate an objective lens closer to the wafer than mirrors or prisms adapted to bend the optical axis of the alignment optical system. Accordingly, the distance or spacing between the objective lens and the surface of the wafer is increased, and thus the aperture of the objective lens must be increased. It is not easy to produce a large-aperture objective lens with a small aberration, and, even if such a lens is produced, it cannot be positioned without interfering with the projection optical system. Thus, it was inevitable for the known apparatus to use an objective lens with a small N.A.
In order to deal with a wide variety of processes, namely, to improve the mark detection accuracy for each layer, the exposure apparatus as described above includes a plurality of alignment systems of different detection types, for example, FIA (Field Image Alignment) type, LIA (Laser Interferometric Alignment) type, and LSA (Laser Step Alignment) type, as disclosed in U.S. Pat. No. 4,962,318 and U.S. Pat. No. 5,151,750. Where the plural alignment systems are incorporated in a single alignment optical system unit such that at least part of the unit is used in common for these systems, the optical path needs to be diverged or split into a plurality of paths by means of a beam splitter(s). As a result, the structure of the optical system becomes complicated, and the quantity of light of an alignment beam is undesirably reduced, which impedes an improvement of the detecting accuracy. Thus, it has not been easy to satisfy both requirements for dealing with a wide variety of process and improving the detecting accuracy at the same time.
In the meantime, the automatic focus sensor as indicated above includes a light transmitting system and a light receiving system, which are located at respective positions that are spaced from the optical axis of the projection optical system. Accordingly, the relative positional relationship between the light transmitting system and the light receiving system tends to vary due to vibrations, thermal deformation, and the like, and the variations in the positional relationship make it difficult to achieve highly accurate detection, and result in a lack of stability. Also, the automatic focusing sensor is inevitably uses an objective lens having a small N.A. or aperture, for the same reason as described above with respect to the off-axis type alignment system.
Thus, the known exposure apparatus suffers from a lot of problems to be solved for improvement in the accuracy with which the position of the wafer is detected.
It is therefore an object of the present invention to provide exposure apparatus and method that assure improved accuracy with which the position of a substrate is detected, and improved accuracy in overlaying the mask and the substrate. In particular, the present invention aims at providing such exposure method and apparatus that permit highly accurate detection of the position of the substrate, so as to enable the substrate to be exposed with an exposure beam through a projection optical system including a reflecting optical element located close to the image plane of the system. It is another object of the invention to provide a method for manufacturing devices, which method includes steps of detecting the position information of the substrate with high accuracy, and transferring a device pattern on a workpiece.
EUV exposure apparatuses using EUV (Extreme Ultra Violet) light having a wavelength in the range of 5-20 nm, for example, 13.4 nm or 11.5 nm, as exposure light have begun to be developed as exposure apparatuses in the next generation, wherein a circuit pattern whose practical minimum line width (design rule) is about 50 nm to 35 nm is transferred onto a substrate (wafer). In the absence of any optical element that can pass EUV light therethrough, the EUV exposure apparatus is required to use a reflection type reticle, and a reflection type projection optical system consisting solely of reflecting optical elements.
The reflection type projection optical system has an arc shaped illumination region generally called xe2x80x9cring fieldxe2x80x9d, and is characterized in that the optical path of an illumination beam is offset with respect to the optical axis of the projection optical system. If all of the reflecting optical elements that constitute the projection optical system are located coaxially about the optical axis of the projection optical system, therefore, most of the reflecting optical elements include unnecessary regions that are not used for the purpose of reflecting the illumination beam, namely, regions that may be eliminated from the beginning or may be cut out. This is also true with the case of a reflection/refraction type projection optical system including refractive optical elements as part thereof, wherein an optical path of an illumination light beam is offset with respect to the optical axis of the projection optical system.
In a scanning exposure apparatus according to the first aspect of the present invention wherein a pattern of a mask is transferred onto a substrate through a projection optical system while the mask and substrate are moved in synchronism with each other, the projection optical system includes reflecting optical elements as at least part of the system, and at least one of the reflecting optical elements has an exposure-light reflecting region that is located at a position spaced or offset from the optical axis of the projection optical system, while a space including the optical path of exposure light is provided on the side of the optical axis with respect to the reflecting region, and a position detecting device for detecting the position of the substrate is located in at least a part of the space. Thus, the position detecting device can be located close to the optical path of the exposure light, namely, the illumination region. With this arrangement, the distance between an objective optical system of the position detecting device and the substrate can be reduced, thus permitting the use of an objective optical system having a large N.A., which leads to improved accuracy with which the position of the substrate is detected, and improved overlaying accuracy between the mask and the substrate.
In a scanning exposure apparatus according to the second aspect of the present invention wherein a pattern of a mask is transferred onto a substrate through a projection optical system while the mask and substrate are moved in synchronism with each other, the projection optical system includes reflecting optical elements as at least a part of the system, and one of the reflecting optical elements that constitute the projection optical system is located optically closest to the mask and physically closest to the substrate, while a space including the optical path of exposure light is present in a part of the above-indicated one reflecting element or an extended part thereof, and a position detecting device for detecting the position of the substrate is located in at least a part of the space. Thus, the position detecting device can be located close to the optical path of the exposure light, namely, the illumination region. With this arrangement, the distance between an objective optical system of the position detecting device and the substrate can be reduced, thus assuring an improved overlaying accuracy between the mask and the substrate.
In the exposure apparatuses according to the first and second aspects of the invention, where the position detecting device is a mark detecting device (such as an alignment optical system) for detecting marks formed on the substrate, for example, the distance between the center of detection of the mark detecting device and the center of projection of the projection optical system (namely, baseline distance) is remarkably reduced, thus assuring improved stability of the mark detecting device. Also, where the position detecting device is a focal position detecting device (focus sensor) for detecting the position information of the substrate as viewed in the direction parallel to the optical axis of the projection optical system, the length of the optical path of detection light is reduced, thus assuring improved the stability and reducing the size of the focal position detecting device.